An apparatus for manufacturing a three-dimensional article by additive manufacturing includes a processor and an information storage device storing software instructions. In response to execution by the processor, the software instructions cause the apparatus to: receive initial data defining the three-dimensional article having an outer surface, define a shell having the outer surface of the three-dimensional article and an opposing inner surface that defines an inner cavity, define a transition zone between the inner surface of the shell and a boundary that is inside the inner cavity and generally follows the inner surface of the shell, define a lattice of arrayed unit cells that fill the inside of the boundary, the lattice is defined by connected lattice segments, and define transition segments that couple the lattice to the inner surface of the shell.